Micromechanical component and corresponding production method

ABSTRACT

A micromechanical component is described which includes a substrate; a monocrystalline layer, which is provided above the substrate and which has a membrane area; a cavity that is provided underneath the membrane area; and one or more porous areas, which are provided inside the monocrystalline layer and which have a doping that is higher than that of the surrounding layer.

FIELD OF THE INVENTION

The present invention relates to a micromechanical component and amanufacturing method for producing it.

BACKGROUND INFORMATION

Membranes are usually manufactured by bulk or surface micromechanics.Bulk micromechanical designs have the disadvantage that they arerelatively complex to manufacture and are therefore expensive. Surfacemicromechanical variants have the disadvantage that it is generally notpossible to manufacture monocrystalline membranes.

Monocrystalline membranes have the advantage that the mechanicalproperties are more defined than in polycrystalline membranes. Moreover,it is possible to manufacture piezoresistive resistors having asignificantly better long-term stability and higher piezoelectriccoefficients using monocrystalline membranes than piezoresistiveresistors in polycrystalline membranes.

SUMMARY OF THE INVENTION

The micromechanical component according to the present invention and thecorresponding manufacturing method for producing the micromechanicalcomponent provide the advantage that a cavern having a superimposedmonocrystalline membrane may be manufactured simply and cost-effectivelyusing surface micromechanics. The monocrystalline membrane may be used,for example, for pressure sensors.

In accordance with the present invention, to manufacture the membrane,n⁺- or p⁺-doped areas are first selectively anodized (porous etching),which is carried out locally by means of a monocrystalline cover layer,e.g., an epitaxial layer. This is followed by a time-controlled switchto selective electropolishing of an n⁺- or p⁺-doped layer buried underthe membrane. In this way, a cavity or a cavern is produced under thecover layer. Optionally, the porous n⁺- or p⁺-doped areas in the coverlayer are finally sealed to enclose a defined gas pressure in the cavityproduced.

Advantages of the present invention include simple integration into asemiconductor circuit process, consequently making it possible, forexample, to integrate a membrane having an evaluation circuit on a chip(e.g., as a pressure sensor). In addition, little fluctuation due tounderetchings occurs, i.e., it is possible to implement exactlyspecifiable dimensions. Moreover, simple sealing of the access openingsis possible, if desired.

According to an exemplary embodiment, one or more sealing layers areprovided above the monocrystalline layer to seal the porous areas.

According to another exemplary embodiment, the porous areas are sealedby oxidation. This is a particularly effective sealing method.

According to another exemplary embodiment, the monocrystalline layer andthe porous areas are of the same doping type.

According to another exemplary embodiment, the monocrystalline layer andthe porous areas are of different doping types.

According to another exemplary embodiment, the monocrystalline layer isprovided by epitaxy.

According to another exemplary embodiment, the substrate is of a firstconduction type and the buried layer is of a second conduction type. Inthe buried layer, one or more areas of the first conduction type areprovided, which have higher doping than the substrate. This makes itpossible to concentrate the lines of force during electropolishing andto avoid undesirable residues in the cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-1 c show cross-sectional views illustrating different stagesof the manufacturing process for a micromechanical component accordingto a first embodiment of the present invention.

FIGS. 2 a and 2 b show cross-sectional views illustrating differentstages of the manufacturing process for a micromechanical componentaccording to a second embodiment of the present invention.

FIG. 3 shows a possible complication in manufacturing themicromechanical component according to the present invention.

FIGS. 4 a and 4 b show cross-sectional views illustrating differentstages of the manufacturing process for a micromechanical componentaccording to a third embodiment of the present invention.

DETAILED DESCRIPTION

Identical reference numerals in the figures denote identical componentsor components having an identical function.

FIGS. 1 a-1 c show cross-sectional views illustrating different stagesof the manufacturing process for a micromechanical component accordingto a first embodiment of the present invention.

In FIGS. 1 a-1 c, reference numeral 1 denotes a p-doped siliconsubstrate; 5 denotes a buried n⁺-doped layer; 10 denotes an n-epitaxiallayer; 15 denotes n⁺-doped areas in n-epitaxial layer 10; 10 a denotes alater membrane area; 20 denotes a mask; and 30 denotes a sealing layerof, e.g., metal, oxide, nitride, BPSG, etc.

According to FIG. 1 a, buried n⁺-doped layer 5 in p-silicon substrate 1is produced under n-doped epitaxial layer 10 by standard process steps,e.g., by implantation. In addition, n⁺-doping areas 15 are incorporatedin epitaxial layer 10 at selected sites, e.g., at points or in the formof strips or rings, to produce n⁺-doped connections from the surface toburied n⁺-layer 5. Optionally, it is possible to deposit and structure amasking layer 20 or a plurality of such masking layers (e.g., ofnitride) on epitaxial layer 1.

According to FIG. 1 b, n⁺-doping areas 15 may be converted into porousn⁺-areas or entirely dissolved by electrochemical etching in solutionscontaining hydrofluoric acid (“anodization”), depending on theanodization conditions (hydrofluoric acid concentration, currentdensity, etc.). The anodization rate is strongly dependent on the dopingof the silicon. Low-doped n-silicon (n-epitaxial layer) is barelyattacked while n⁺-doped silicon is readily attacked. This selectivity isused to advantage in this embodiment.

In a first, time-controlled anodization step, n+-doped areas 15 inepitaxial layer 1 are etched to more or less complete porosity. Theporosity is preferably greater than 50%. A change of the anodizationconditions causes buried n+-doped layer 5 to be dissolved away when anetchant penetrates through the now porous areas 150 to the buriedn+-doped layer 5. In the transitional area from n+-area 5 to p-dopedsubstrate 1, there is a weakly n-doped area which acts as an anodizationlimit. The form of buried n+-doping 5 defines the area dissolved out.

According to FIG. 1 c, porous areas 150 may—if desired—be closed verysimply in a subsequent process step after removal of mask 20 becausethey have a nearly flat surface having very small holes. This is asignificant advantage compared to standard micromechanical surfacemanufacturing methods in which holes usually having diameters greaterthan one μm must be sealed. The sealing may be performed, for example,by deposition of metal layer 30 or several layers (oxide, nitride, metalBPSG, . . . ) or by oxidation. The process pressure during depositiondefines the internal gas pressure in cavity 50.

FIGS. 2 a and 2 b show cross-sectional views illustrating differentstages of the manufacturing process for a micromechanical componentaccording to a second embodiment of the present invention. The dopingsused for the etched areas have been varied in this case.

In addition to the reference symbols already introduced in FIGS. 2 a and2 b, 5′ denotes a buried p⁺-doped layer and 15′ denotes p⁺-dopedfeed-through areas.

In this example, a p⁺-doping is incorporated in p-substrate 1 for buriedlayer 5′. In addition, n-epitaxial layer 10 is grown epitaxially over itand provided with p⁺-feedthroughs 15′.

According to FIG. 2 b, p⁺-doped area 15′ is selectively anodized to formporous p⁺-doped area 150′. n-epitaxial layer 10 is not attacked in thiscase, and p-substrate 1 is attacked only slightly since the anodizationrate of p⁺ is significantly higher than that of p and n.

FIG. 3 shows a possible complication in manufacturing themicromechanical component according to the present invention.

When the buried doping layer 5′ is dissolved out via an etchantpenetrating through the porous areas 150′, there is the danger that asilicon web 151 will remain at the point at which two etch fronts meet.This web 151 could cause membrane 10 a not to be completely freed, thusadversely affecting its function.

FIGS. 4 a and 4 b show cross-sectional views illustrating differentstages of the manufacturing process for a micromechanical componentaccording to a third embodiment of the present invention.

In the third embodiment, the danger described in connection with FIG. 3may be confronted by providing a buried p+-doping area 5″ in buriedn+-layer 5 where the etch fronts meet during the subsequent anodization.This p+-doping area 5″ causes lines of force S to be guided selectivelyduring the subsequent anodization so that no web remains after thesilicon is dissolved out via an etchant penetrating through the porousareas 150 to the buried n+-layer 5.

Although the present invention was described on the basis of exemplaryembodiments, it is not limited to them but instead may be modified invarious ways.

The described and illustrated embodiments are only exemplary of themanufacturing sequence. Optionally, additional dopings may beimplemented next to the membrane or in the membrane, for example, tomanufacture piezoresistors in the membrane and an evaluation circuitnext to the membrane for an integrated pressure sensor. The buriedn⁺-doped layer and the n⁺-doped feeds through the epitaxial layer may bedesigned in such a way that the buried layer is dissolved out throughlateral n⁺-etch channels, which are connected with the surface of theepitaxial layer at the channel end via the n⁺-feeds.

1. A method of manufacturing a micromechanical component, comprising:providing a substrate; providing a buried layer on the surface of thesubstrate, the buried layer having higher doping concentration than thesurrounding substrate; providing a monocrystalline layer above thesubstrate; providing inside the monocrystalline layer at least one areahaving higher doping concentration than the surrounding monocrystallinelayer; selectively porous etching the at least one area having higherdoping concentration to produce corresponding at least one porous area;and forming a permanent cavity under the monocrystalline layer and amembrane area above the permanent cavity by dissolving out the buriedlayer, wherein the dissolving out the buried layer to form the permanentcavity takes place entirely through the at least one porous areapreviously produced, by an electropolishing process in which an etchantis introduced, via penetration of the at least one porous area, to thearea to be dissolved, wherein the at least one porous area is part ofthe membrane area above the permanent cavity.
 2. The method as recitedin claim 1, wherein at least one sealing layer is provided above themonocrystalline layer to seal the at least one porous area.
 3. Themethod as recited in claim 1, wherein the at least one porous area issealed by oxidation.
 4. The method according to claim 1, wherein themonocrystalline layer and the at least one porous area are of the samedoping type.
 5. The method as recited in claim 1, wherein themonocrystalline layer and the at least one porous area are of differentdoping types.
 6. The method as recited in claim 1, wherein themonocrystalline layer is provided by epitaxy.
 7. The method as recitedin claim 1, wherein the substrate has a first doping and at least aportion of the buried layer has a second doping different from that ofthe first doping, and wherein at least one area having the same dopingas the substrate and a higher doping concentration than the substrate isprovided in the buried layer.
 8. The method as recited in claim 1,wherein the electropolishing process involves complete dissolution ofmaterial in the buried layer.
 9. The method as recited in claim 8,wherein the dissolution of material in the buried layer is dependent onanodizing conditions.
 10. A method of manufacturing a micromechanicalcomponent, comprising: providing a substrate; providing a buried layeron the surface of the substrate, the buried layer having higher dopingconcentration than the surrounding substrate; providing amonocrystalline layer above the substrate; providing inside themonocrystalline layer at least one area having higher dopingconcentration than the surrounding monocrystalline layer, wherein athickness of the at least one area substantially extends across theentire thickness of the monocrystalline layer; selectively porousetching the at least one area having higher doping concentration toproduce corresponding at least one porous area; and forming a permanentcavity under the monocrystalline layer and a membrane area above thepermanent cavity by dissolving out the buried layer, wherein thedissolving out the buried layer to form the permanent cavity takes placeentirely through the at least one porous area previously produced byintroducing, via penetration of the at least one porous area, an etchantto the area to be dissolved, wherein the at least one porous area ispositioned at a lateral edge of the membrane area above the permanentcavity.